A Boiling Water Reactor (BWR) typically has a core formed of several hundred fuel bundle assemblies. Each fuel bundle assembly is formed of several fuel assembly rods, and each fuel assembly rod contains a variety of radioactive elements. Typically a number of different fuel bundle assemblies are created, and a core designer creates a core design using the different fuel bundle assemblies. The core design involves establishing the positioning of each bundle in the core, which is called the loading map, and establishing an operation plan over a period of time referred to as the operation or loading cycle. The operation plan establishes the operating parameters of the core over the loading cycle. Operating parameters (also referred to interchangeably herein as “independent control variables” and “design inputs”) include, for example, various physical component configurations and controllable operating conditions that may be individually adjusted or set (e.g., control rod positions and adjustments over time—typically referred to as the rod pattern). The loading cycle is typically designed for 1 year, 1.5 years or 2 years. Once a core design has been created, the core design must be licensed by NRC before being put into operation.
Once licensed, the core design can be put into practice. The start of a period of operation is typically referred to as BOC, Beginning of Cycle. When the reactor starts, flow and power are slowly increased over a period of a couple days. Eventually, the reactor is said to be at rated operating conditions (rated power, flow, inlet enthalpy, core pressure, etc.). Here, the reactor will stay at rated operating conditions. After a period of time ranging from a few days to several months the reactor must change the operating or control rod patterns in order to recalibrate for changing core reactivity or in order to operate in an alternative rod pattern strategy or sequence. In order to minimize duty on the fuel bundle assemblies, power is typically reduced during these rod pattern modifications. Hence, additional power ascensions are required after each rod pattern modification. This method of operation occurs for the loading cycle (1 year, 1.5 year, or 2 year periods) until the end of reactivity and operation occurs. The ending of a plants loading cycle is typically referred to as EOC, End of Cycle.